Someone has tried to upgrade from jessie to stretch the LMDE, I tried it but it gave me problems with the cinnamon desktop, I stopped running the firefox, and when I reboot the computer stopped running the X.
Any idea how to do it?

LMDE 2 is not ready for Stretch package base. You may be able to get something working somehow, but software from the LMDE 2 repositories needs work before it will run on Stretch package base. That's called LMDE 3 and isn't here yet